The present invention is directed generally to attaching electrical components to printed circuit boards. The invention relates more particularly to removably affixing surface mount integrated circuit packages with splayed leads to printed circuit boards, wire boards, or wire assemblies.
An integrated circuit combines numerous active and passive electrical circuitry elements on a single device called a die or chip. Commonly, the electronics industry, including the computer, communications, and consumer electronics sub-industries, interconnect integrated circuits by attaching them to printed circuit boards. Because chips are inherently small and fragile devices, they are usually imbedded in a substrate called a chip carrier or package before they are attached to the printed circuit board. Protruding from the package are a number of electrical conducting leads. The manner in which the leads protrude vary; they may extend through the bottom of the package (e.g. pin or pad grid arrays), they may be arrayed along two edges of the package (e.g. dual in-line pins), or they may fan out from the edges of the package (e.g. gull wing and J pins).
The wiring on printed circuit boards comprises thin metallic strips embedded in an insulating material. These strips interconnect leads protruding from one integrated circuit package to leads protruding from other circuit packages mounted on the same board. To make all the required connections, the boards typically have several layers of interconnect wiring. The wiring determines the placement of the integrated circuit packages on the board and routes electrical signals among the integrated circuits. Leads may connect to the wiring in a variety of ways. One method involves drilling holes in the board and through the wiring at appropriate locations, inserting leads through the holes, and making mechanical and electrical attachments among the leads, the wiring, and the holes. Another, increasing popular technique, is called surface mount technology. This method involves arranging contact pads on the surface of the printed circuit board. The pads provide paths for electrical signals from leads to the appropriate embedded wires. Leads are placed on top of the pads and mechanically and electrically attached.
There are a number of methods of mechanically and electrically attaching integrated circuit package leads to printed circuit boards. The most commonly used techniques in the electronics industry require lead based solder. There are many environmental and economic disadvantages in using solder:
Lead is a well known hazardous material linked to numerous serious human ailments. While there is no evidence that lead in electrical solder has an impact on worker health or the environment, both the Environmental Protection Agency and members of Congress have expressed concern. Industry research in this area is ongoing. PA1 Depositing lead onto printed circuit boards often requires fluxes which must be cleansed. The preferred method of cleansing involves freon which is known to persist for long periods in the atmosphere and is known to deplete ozone. Other methods of cleansing boards involve using water which creates problems of treating waste water to eliminate pollutants. PA1 The heat required to deposit solder on boards may damage the components being attached or the board itself. PA1 Repairing a board or component requires reheating which may cause further damage and require scrapping an expensive assembly. PA1 Equipment to mass produce soldered boards (e.g. wave solder and flux cleansing machines) is expensive and cumbersome. PA1 Metal solders frequently short (or bridge) leads to one another, adversely alter the electrical characteristics of integrated circuits (e.g. increasing capacitance between leads), limit how closely leads may be spaced (thus limiting the density of integrated circuits on a board), and result in defective connections (e.g. cold solder joints) which may be difficult or impossible to detect before a board is placed in service. PA1 Soldered bonds between boards and components may be broken when the board is dropped, flexed, or otherwise vibrated. PA1 Many soldered assemblies require gold connections instead of less expensive metals with superior or nearly equivalent electrical properties (i.e. silver, copper, and aluminum), because of gold's thermal expansion and anti-corrosive properties.
Surface mount technologies generally involve applying a 3- to 4-mil coating of solder paste onto contact pads on the surface of the board, placing package leads onto the pads, and melting (reflowing) the solder. This is sufficient mechanically to hold the package in place while making the proper electrical connection. Surface mounting has a number of advantages over older techniques; it saves board space by permitting dies to be mounted in small packages with closely spaced leads, it reduces the number of levels of embedded wires in a board, and it allows components to be mounted on both sides of a board. However, surface mount technology not only entails the known problem of using solder, but also raises the additional problem of properly aligning the leads onto the contact pads. Proper placement of surface mount components generally requires specialized computer controlled equipment.
Because of the advantages of surface mount technology, much research has been directed at the soldering and alignment problems. Two alternatives to soldering are the subject of most research; attaching packages to boards with adhesive (e.g. TAB technology) or applying sufficient pressure to packages to make a mechanical and electrical connection. Proper placement of components for TAB and pressure mount techniques has proved to be as difficult a problem as it is for solder techniques.
Most pressure techniques involve compressing the package itself to the board. A compressible pad with embedded electrically conductive material, known as a Z-axis connector, is frequently placed between the package and the board. Usually only leadless packages with contact pads on the bottom surface may be used; pressure on the top of a package would stress the point at which splayed leads, such as gull wings, attach to the package. In any event, applying pressure to the top of any package sufficient to maintain a proper electrical connection between the leads and the board contact pads, may damage the package.